1. Field of the Invention
The present invention relates to glass fibers. More preferably, the present invention relates to strong glass fibers having a permanent twist to thereby offer improved loft and recovery. The present invention relates to such novel glass fibers which are comprised of two glasses in a core/sheath relationship and a method of making same.
2. Description of the Related Art
Small diameter glass fibers are useful in a variety of applications including acoustical or thermal insulation materials. When these small diameter glass fibers are properly assembled into a lattice or web, glass fibers which individually lack strength or stiffness can be formed into a product which is quite strong. The glass fiber insulation which is produced is lightweight, highly compressible and resilient. For purposes of this patent specification, the term xe2x80x9cglassxe2x80x9d is intended to include any of the glassy mineral materials, such as rock, slag and basalt, as well as traditional glasses.
In the shipping and packaging of glass products, high compressibility is required. It is desirable to compress the glass products for shipping and then have them recover rapidly and reliably to the desired size. For example, to economically ship high-efficiency air-filter media from U.S. production operations to foreign countries, it is desirable to compress the media as tightly as possible to maximize the number of units in a shipment. When unpackaged, however, the product must recover to meet the high filtration efficiencies and other properties customers expect. The same is true for insulation products. Current insulation products are limited in the amount of compression possible while still attaining adequate recovery. When the product is compressed, the binder holds firm while the fibers themselves flex. As the stress upon the fiber increases due to excessive compression, the fiber breaks.
Attempts have been made in the prior art to produce non-straight glass fibers to thereby provide loft and resiliency in the glass products. In a mechanical kink process, glass fibers are pulled from a textile bushing. While still at high temperatures, the fibers are pulled by mechanical means through a series of opposed gears or a crimping device to attenuate and crimp them. The net result is a bundle of kinked glass fibers.
The major disadvantage to mechanical kinking is that the fibers are not conducive to satisfactory glass mat production. Every fiber produced in this manner has a uniform shape, defeating the purpose of the kink. Further, the process has an unsatisfactory low throughput.
U.S. Pat. No. 2,998,620 discloses curly glass fibers of bicomponent glass compositions. The patent discloses producing staple curly fibers by passing two glass compositions of differing thermal expansivity through the orifices of a spinner. The glasses are extruded as a dual glass stream in aligned integral relationship such that the fibers curl naturally upon cooling due to the differing thermal expansivity.
U.S. Pat. No. 5,536,550 describes the manufacture of bicomponent curly glass fibers by co-fiberization of two distinct glasses having coefficients of thermal expansion (CTE) which are sufficiently different that upon cooling, the internal stress created by the differences in the CTEs of the glasses cause attenuated fibers to curl. To generate different CTEs, different glass chemistries are used.
U.S. Pat. No. 3,073,005 discloses a non-rotary process for making bicomponent curly glass fibers. The fibers are made by feeding differing glass compositions to an orifice in side by side contact such that the two glasses are attenuated into a single fiber.
U.S. Pat. No. 2,927,621 also discloses the production of curly fibers. The patent discloses a process wherein glass fibers of a single glass composition are passed through opposed contoured skirts after the fibers have been softened by hot gases. The fibers then take on the shape of the contour of the skirts.
U.S. Pat. No. 5,529,596 discloses a method for making dual-glass fibers by causing one glass to flow around another glass as they are spun from a rotating spinner. The dual-glass fibers are formed from glasses having differing softening points. The glass mat prepared is passed through an oven to heat the web to a temperature greater than the softening point of the lower softening point glass, but less than the softening point of the second glass. This enables the one glass to soften and act as a binder, bonding the glass fibers to each other.
U.S. Pat. No. 5,629,089 discloses a dual glass fiber insulation product, which includes irregularly-shaped glass fibers. The glass fibers are made of two different glasses having different coefficients of thermal expansion. A cross-section of the fibers shows the two glasses side-by-side, with an interface between the two glasses. Because the two glasses have different coefficients of thermal expansion, one glass will contract more than the other on cooling, causing the fiber to bend. The irregular shaped fiber differs from a helical fiber in that the rotation of the fiber is not constant, but rather varies irregularly both in direction and in magnitude. See also U.S. Pat. No. 6,017,835.
While the prior art has provided methods and glass fibers appropriate for rendering glass products resilient, improvements are still needed. The past twenty years have seen major restrictions placed on the properties of glasses used to make glass fibers. Material and operational costs have narrowed the number of major glass components down to less than ten, and concerns over health and safety have placed additional restrictions on the levels of these components in glass formulae. At the same time, the demand for better product performance has not softened; if anything, it has increased. Furthermore, the factions of health and safety are frequently at odds with product performance. The U.S. and European markets are especially sensitive to the use of what it deems as potentially hazardous materials in air filtration and other applications. Improving the capabilities of fiberglass products while trying to make the glass more biosoluble involves compromises in glass chemistry, compromises that generally make a new glass weaker structurally than fiberizable glass used in the past. Moreover, there are more expensive, but seemingly less hazardous, competitive materials waiting in the wings for the opportunity to take market share from many fiberglass products. It is, therefore, desirable to invent a glass fiber that offers a combination of safety and improved product performance, without major modifications to existing processing methods.
Accordingly, it is an object of the present invention to provide a novel glass fiber which exhibits good product performance while also offering good health and safety properties.
It is another object of the present invention to provide a process for preparing such novel glass fibers.
It is yet another object of the present invention to provide a novel glass fiber which exhibits good biosolubility, good strength, while also having a permanent twist or curl to impart improved loft and recovery.
These and other objects of the present invention will become apparent on review of the following specification, the figures of the drawing, and the claims appended hereto.
The glass fiber of the present invention involves the combining of two glasses, preferably at least one of which is biosoluble in a core/sheath arrangement. Such an arrangement can provide fibers potentially stronger than those of single glasses, while also having a permanent twist or curl to impart impoved loft and recovery to blankets or mats comprised of them. In one embodiment, the glass fibers of the present invention contain a core and a sheath, where the core has a lower Tg so that the core glass would independently tend to shrink more on cooling than the sheath glass. Benefits are achieved in this manner, preferably with regard to biosoluble fibers.
In preparing the fibers of the present invention, one glass is entrained within the second during fiber formation, or in a process step prior to fiberization. The core/sheath relation occurs regardless of glass properties as the outer glass is forced around the inner core in the process of the present invention. The hole sizes used in the formation are selected to allow the glasses to flow properly, taking into account the glass properties.
The entrained, or xe2x80x9ccorexe2x80x9d, glass preferably has a tendency to shrink more on cooling than the outer, or xe2x80x9csheathxe2x80x9d glass. This differential tendency for shrinkage puts the outer glass under compression, making the fiber stronger than it would be if comprised of the sheath glass alone. Moreover, as the outer glass is placed under compression, the core glass is put under tension. It is believed that such thermally stressed fibers, if inhaled, are more susceptible to breakdown within the lungs than non-stressed fibers. In vitro tests on similar fibers show a two-phase mechanism of fiber dissolution, a chemical leaching phase followed by a mechanical collapse of the structure. It is believed that body fluids would attack the stretched bonds of the glass and remove certain components. The remaining bonds relax and, in the process, distort and crack the weakened structure of the fiber.
Permanent curl is preferably provided by offsetting the longitudinal axis of the cylinder of core glass from the longitudinal axis of the fiber. In this case, the fiber is under more compression on the side closest to the core glass than the opposite side, which differential causes the fiber to curl. Imparting permanent curl in such a manner also aids in the biosolubility of the glass fiber.
In another embodiment, there is provided a novel method for preparing the core/sheath glass fibers of the present invention. The core/sheath and offset core/sheath fibers can be manufactured via both pot-and-marble flame-attenuated and rotary spinning processes. Many benefits are realized by the core/sheath and offset core/sheath fibers of the present invention relative to the other kinds of curly fibers.